Electric Power Assisted Steering (EPAS or EPS) systems employ an electric motor to assist the driver in providing the amount of torque required to control the direction of the wheels of a vehicle for steering the vehicle. Sensors disposed within the vehicle are used to detect the motion and torque of the steering wheel. A computer module calculates an amount of assistive torque to apply and instructs an electric motor to apply that assistive torque.
In closed-loop EPAS systems a desired driver exerted steering torque (also referred to as reference torque or target torque) is calculated from input parameters and variables. In a closed-loop EPAS system, the actual driver-applied torque is compared to the desired reference driver torque and the electric motor that provides the assistive power is controlled to minimize the difference between the desired reference driver torque and the actual driver applied steering torque. In a closed-loop system the steering characteristics that are experienced by the driver are controlled. A computed desired reference driver torque is typically dependent upon variables such as the vehicle speed, the vehicle direction (forward travel or revere travel) and the steering wheel angle, which are received in real-time.
Such closed loop control systems must artificially synthesise all the desirable elements that make up the steering feel and as such usually comprise dedicated algorithms that are included to generate, among other things, an artificial friction component of the desired reference torque. Those algorithms are needed because the closed loop system, could otherwise completely cancel out the torques due to friction and hysteresis that are normally felt in the steering system and without the artificial friction or artificial hysteresis, the steering characteristics experienced by a driver could feel “artificially clean” and/or could make the steering control task harder for the driver.
One such closed-loop EPAS system is disclosed in WO2010/089172 to ZF Lenksysteme GMBH. In WO '172 it is disclosed that in order to implement a steering feel having a control concept such that it can be adapted to different steering systems, a target steering torque (torTB) is computed as a function of: a base steering torque (torB), a damping torque (torD), a hysteresis torque (torF) and a centring torque (torCF; torC). The base steering torque (torB) is determined as a function of a force acting externally (torR) and a vehicle speed (velV). The damping torque (torD) is determined as a function of the steering speed (anvSW) and the vehicle speed (velV). The hysteresis torque (torF) is determined as a function of the steering speed (anvSW) and the vehicle speed (velV). The centring torque (torCF; torC) in the direction of the straight line position is determined as a function of the steering wheel angle (angSW) and the vehicle speed (velV).
The quality of steering feel is considered by the applicant to be a critical characteristic and it is desirable to refine the steering feel of known closed-loop EPAS systems.
It is an object of the present invention to provide a closed-loop EPAS system and a vehicle comprising the same that exhibits refined or tuned steering characteristics compared to current closed-loop EPAS systems. More, specifically, it is an aim to improve the characteristics of reaction torque. This is because if the driver experiences a good reaction in torque in the steering as soon as the steering wheel is turned, a sense of confidence and a “well connected” feel may be imparted to the driver. This may enhance the driving experience. As such in the EPAS system of the present disclosure, a target steering torque (also referred to as a desired reference torque (τref)) is computed that incorporates an hysteresis torque component (also referred to as an artificial friction component) that is dependent upon steering wheel angle (α).